Electronic circuit with device for monitoring a power supply

ABSTRACT

A power supply voltage is monitored by a monitoring circuit including a band gap voltage generator core including a first node and a second node. A control circuit connected to the first and second nodes is configured to deliver a control signal on a first output node having a first state when an increasing power supply voltage is below a first threshold and having a second state when increasing power supply voltage exceeds the first threshold. The first threshold is at least equal to the band gap voltage. An equalization circuit also connected to the first and second nodes with feedback to the band gap voltage generator core generates the bandgap voltage at a second output node. The control signal operates to control actuation of the equalization circuit.

PRIORITY CLAIM

This application claims the priority benefit of French Application forPatent No. 1759915, filed on Oct. 20, 2017, the content of which ishereby incorporated by reference in its entirety to the maximum extentallowable by law.

TECHNICAL FIELD

Embodiments relate to integrated circuits, and in particular to managingthe start-up and operation of the integrated circuits as a function ofthe power supply voltage.

BACKGROUND

In order to avoid integrated circuit malfunctions, there are devicesthat make it possible to delay the start-up of an electronic circuituntil the power supply voltage of the circuit has reached a firstthreshold.

These devices can also switch off the integrated circuit if the powersupply voltage falls below a second threshold, the second thresholdconventionally being able to be equal to or lower than the firstthreshold.

These devices are known to the person skilled in the art as “Power OnReset circuits” (POR circuits).

However, the existing systems present certain drawbacks, in particularan excessive error margin, of the order of 10 to 20%. This error margincan moreover vary as a function of the temperature, of the voltagethreshold, or of the technology of the integrated circuit.

There is therefore a need to improve this type of device.

SUMMARY

Thus, according to one embodiment, a device is proposed for monitoringthe power supply of an integrated circuit, having a high level ofaccuracy and insensitive to change of temperature, of voltage thresholdor of fabrication method.

According to one aspect, an electronic circuit is proposed comprising apower supply node configured to receive a power supply voltage, a firstoutput node and a reference node configured to receive a referencevoltage, and comprising a device for monitoring the power supplyvoltage.

The monitoring device comprises a band gap voltage generator corecoupled to the power supply node via a voltage power supply moduleconfigured to supply the core with voltage, the core comprising a firstnode and a second node.

The monitoring device further comprises a control circuit connected tothe two nodes of the core and to the first output node.

The core and the control circuit are configured so that the controlcircuit delivers, on the first output node, a control signal having afirst state when the power supply voltage increases and remains below afirst threshold, and a second state when the power supply voltagebecomes greater than or equal to the first threshold, the firstthreshold being at least equal to the band gap voltage.

When the voltage at the power supply node reaches the first threshold,the voltages at the two nodes of the core are equal. As a function ofthe structure of the current power supply, the first threshold can beequal to the band gap voltage (typically 1.2 Volts) or greater than thisband gap voltage, for example between 1.2 Volts and 1.4 or 1.5 Volts.

A voltage of 1.2 Volts can therefore be advantageously used in somecases as the voltage threshold of the device.

Furthermore, the core allows for a particularly accurate detection ofthe voltage threshold, being insensitive to the variations oftemperature, of fabrication method or of voltage threshold.

The voltage power supply module can comprise the power supply nodecoupled to an intermediate node through which the core is supplied withvoltage.

The circuit can be configured so that the intermediate node receives thepower supply voltage, the first threshold being equal to the band gapvoltage. Such is the case when the intermediate node is directly coupledto the power supply node, or else coupled to the power supply node, forexample, via a follower amplifier.

That said, the voltage power supply module can comprise a resistivemodule coupled between the power supply node and the intermediate node,the first voltage threshold then being greater than or equal to the bandgap voltage.

Thus, it is advantageously possible to modify the first voltagethreshold by adjusting the resistance value of the resistive module.

A core structure that is particularly simple to produce and small interms of surface area uses PNP transistors and three resistors. Thispreferred example that makes it possible to easily reduce the firstvoltage threshold to the band gap voltage by dispensing with theresistive module is however of course non-limiting, other corestructures being able to be envisaged.

More specifically, according to this non-limiting example, the corecomprises two branches respectively coupled to the two nodes andrespectively comprising diode-mounted PNP bipolar transistors configuredto exhibit different current densities, the branch exhibiting thegreatest current density further comprising a first resistor. The corealso comprises two auxiliary resistors respectively connected to the twonodes of the core and having a common node forming said intermediatenode.

The control circuit can be configured to deliver the control signal inits first state when the power supply voltage drops back below the firstthreshold.

According to one embodiment, the control circuit comprises a firstcomparator of which a first input is coupled to the first node, of whicha second input is coupled to the second node, and of which the output iscoupled to the first output node.

According to one embodiment, the circuit can comprise a band gap voltagegenerator incorporating the core, having an equalization circuit thatcan be activated by the control signal, and a second output node, theequalization circuit being deactivated when the control signal is in itsfirst state, and activated when the control signal is in its secondstate, the second output node being able to deliver the band gap voltagewhen the equalization circuit is activated.

Thus, one and the same core is used for the device and for the band gapvoltage generator, which advantageously allows for a saving on surfacearea compared to a circuit in which the device and the band gap voltagegenerator would each use a distinct core.

It should be noted here that this aspect providing for the use of thesame core for, on the one hand, a band gap voltage generator withactivatable equalization circuit and for, on the other hand, a devicefor monitoring the power supply voltage, is compatible with any corestructure, regardless of the value of the first threshold, whether thisvalue is equal to the band gap voltage or greater than this band gapvoltage.

According to one embodiment, the control circuit comprises a secondcomparator of which a first input is coupled to the first node, of whicha second input is coupled to the second node, and of which the output isconfigured to generate a first signal having a first state when thepower supply voltage is greater than or equal to the first voltagethreshold, and a third comparator of which a first input is coupled tothe second output node, of which a second input is coupled to the powersupply node, and of which the output is configured to generate a secondsignal having the second state when the power supply voltage is greaterthan or equal to the first threshold, the device further comprising alogic gate of AND type of which a first input is coupled to the outputof the second comparator, of which a second input is coupled to theoutput of the third comparator via an inverting logic gate, and of whichthe output is coupled to the first output node.

That advantageously makes it possible to generate a control signal whenthe power supply voltage drops back below the voltage threshold, despitethe presence of the amplifier of the band gap voltage generator whichhas a higher gain than that of the first comparator.

The control circuit can be configured to deliver the control signal inits first state when the power supply voltage drops back below a secondthreshold lower than the first threshold.

The second input of the second comparator can be coupled to the powersupply node via a voltage divider bridge, so that the third comparatoris configured to generate the second signal in the first state when thepower supply voltage is greater than or equal to a second voltagethreshold, the second voltage threshold being lower than the firstvoltage threshold.

The use of voltage thresholds which are different in the rising and inthe falling of the power supply voltage confers a hysteresis-likebehavior on the device and advantageously makes it possible to avoidoscillation phenomena which could occur with the use of one and the samethreshold.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 to 5 show embodiments for an electronic circuit comprising adevice for monitoring a general power supply.

DETAILED DESCRIPTION

FIG. 1 illustrates an electronic circuit CI comprising a device DIS formonitoring the general power supply of the circuit CI, configured togenerate a control signal CTRL at an output node BS, intended for theintegrated electronic circuit CI.

The state of the control signal CTRL changes as a function of the valueof the general power supply voltage Vcc of the circuit CI, and allowsthe circuit CI to operate only if the power supply voltage Vcc is abovea first threshold that is here, for example, 1.2 Volts.

The control signal CTRL can be in a first state, for example a lowstate, or in a second state, for example a high state, and the circuitCI is configured to operate when the control signal CTRL is in itssecond state, and to be switched off when the control signal CTRL is inits first state.

The device DIS is configured to generate the control signal CTRL havingthe second state when the voltage at the power supply node BV is greaterthan or equal to the first voltage threshold.

Here, the general power supply voltage Vcc of the integrated circuit CIis supplied by a battery (not represented), coupled to a power supplynode BV of the integrated circuit CI.

The device DIS is coupled to the power supply node BV via a voltagepower supply module MA configured to deliver the power supply voltageVcc to an intermediate node NI. The voltage power supply module hereonly comprises the power supply node BV.

The device DIS comprises a core circuit CR, comprising a first node BE1and a second node BE2.

The core circuit CR here comprises a first PNP bipolar transistor,referenced Q1, diode-mounted and connected in series with a firstresistor R1, here with a value of 1 mega ohm, between the first node BE1and a reference node BR, intended to be supplied by a reference voltage,here the ground. The first transistor Q1 connected in series with thefirst resistor R1 here forms a first branch BR1 of the core circuit CR.

The core circuit CR also comprises a second PNP bipolar transistorreferenced Q2, also diode-mounted, and connected between the second nodeBE2 of the core circuit and the reference node BR. The second transistorQ2 coupled between the second node BE2 and the reference node BR hereforms a second branch BR2 of the core circuit CR.

The size of the first transistor Q1 and the size of the secondtransistor Q2 are different, and are in a surface ratio M, such that thecurrent density passing through the second transistor Q2 is M timesgreater than the current density passing through the first transistorQ1.

For example here, the size of the first transistor Q1 is eight timesgreater than the size of the second transistor Q2.

Obviously, it would also be possible to use a transistor Q2 and Mtransistors Q1 in parallel, all of the same size as the secondtransistor Q2.

The core circuit CR further comprises a first auxiliary resistor Rx1coupled between the intermediate node N1 and the first node BE1, and asecond auxiliary resistor Rx2 coupled between the intermediate node N1and the second node BE2. Here, the first auxiliary resistor Rx1 and thesecond auxiliary resistor Rx2 both have a value of 10 mega ohms.

The core circuit CR of the device DIS is here analogous to a band gapvoltage generation device circuit.

As is known to the person skilled in the art, a band gap voltagegenerator conventionally comprises, in addition to a core circuitanalogous to the core circuit CR described previously, an equalizationcircuit comprising an amplifier and a feedback stage configured toequalize the voltage at the first node BE1 and at the second node BE2 ofthe core circuit CR. And, when the voltage at the first node BE1 isequal to the voltage at the second node BE2, the voltage at theintermediate node NI is a band gap voltage equal to the sum of thevoltages at the first node BE1 and at the second node BE2,conventionally 1.2 Volts.

A band gap voltage is independent of temperature.

Thus, the equalization circuit makes it possible to keep the voltagesequal at the first and second nodes BE1 and BE2, and therefore maintaina band gap voltage at the intermediate node NI.

Specifically, it is known to the person skilled in the art that, whenthe voltages at the first and second nodes BE1 and BE2 are equal, thevoltage at the intermediate node NI is a voltage equal to 1.2 Volts.

That is possible with a ratio between the value of the first resistor R1and the value of the auxiliary resistors Rx1 and Rx2 of the order of 10.For example, the first resistor R1 has a value of 1 mega ohm.

Here, the device DIS does not comprise a feedback stage or an amplifier,the voltages at the first node BE1 and at the second node BE2 are nottherefore kept equal.

The device DIS comprises control circuit MC, here comprising a firstcomparator CMP1 of which a first input E11 is coupled to the first nodeBE1, a second input E12 is coupled to the second node BE2, and of whichthe output is coupled to the output node BS of the device DIS.

The first comparator CMP1 is here configured to generate the controlsignal CTRL in its second state when the voltage at the first node BE1is greater than or equal to the voltage at the second node BE2.

In other words, the first comparator CMP1 generates the control signalCTRL having its first state when the power supply voltage has reachedthe first threshold.

In operation, on starting up the general power supply, the voltage Vccincreases progressively to its maximum value.

In a first phase, the voltage Vcc is lower than the first threshold, andthe voltage at the first node BE1 is lower than the voltage at thesecond node BE2.

As the power supply voltage Vcc approaches the first threshold, thedifference between the voltage at the first node BE1, that is to say onthe first input E11 of the first comparator CMP1, and the voltage at thesecond node, that is to say on the second input E12 of the firstcomparator CMP1, decreases.

A second phase begins when the value of the power supply voltage Vccreaches the first threshold, here a band gap voltage valueconventionally equal to 1.2 Volts. The voltages at the nodes BE1 and BE2are then equal.

The power supply voltage Vcc continues to increase from the firstthreshold to its maximum value, at which it stabilizes. In this secondphase, the voltage at the first node BE1 is therefore equal, thengreater than the voltage at the second node BE2.

The first comparator CMP1 therefore delivers the control signal CTRLwhich therefore takes its second state and the electronic circuit CIstarts its operation.

And, during the operation of the integrated circuit CI, it is possible,because of a malfunction, for the power supply to drop abruptly. In thecase where the power supply voltage Vcc drops below the first threshold,that is to say also if the voltage at the first node BE1 once againbecomes lower than the voltage at the second node BE2, the firstcomparator CMP1 delivers the control signal CTRL in its first state andthe integrated circuit CI stops its operation.

It is therefore possible here to ensure that the integrated circuit CIdoes not operate if the power supply voltage Vcc is too low.

Furthermore, the use of the core circuit CR, analogous to a band gapvoltage generator core, makes it possible to obtain the first thresholdwith a high level of accuracy.

As an indication, the Inventors have observed that the value of thefirst threshold is respected with a margin of error of the order of 1 to3%.

In order to modify the value of the first voltage threshold, it wouldalso be possible, as illustrated in FIG. 2, for the voltage power supplymodule MA to comprise a follower amplifier AS, configured to deliver thepower supply voltage Vcc to the intermediate node NI, and a resistivemodule, here a first voltage divider bridge DIV1 comprising a secondresistor R2 coupled between the power supply node BV and the followeramplifier AS and a third resistor R3 coupled between the second resistorR2 and the ground.

The follower amplifier AS advantageously makes it possible to isolatethe device DIS of the resistive module in terms of current.

It would however be possible for the voltage power supply module tocomprise only the power supply node BV and the resistive module.

It is thus possible, as a function of the value of the second resistorR2 and of the third resistor R3, to choose the first voltage thresholdat the power supply node.

That said, it should be noted that this voltage threshold cannot belower than 1.2 Volts, that is to say the first threshold when the valueof the second resistor R2 is zero.

In other words, if the second resistor R2 and the third resistor R3 arezero, the circuit will start its operation when the voltage Vcc reachesa value of 1.2 Volts.

If the resistors R2 and R3 are not zero, the circuit will start itsoperation when the power supply voltage Vcc reaches a value greater than1.2 Volts, this value depending on the values of the second and thirdresistors R2 and R3. More specifically, this value of the firstthreshold is equal to 1.2*(R2+R3)/R3.

FIG. 3 illustrates an embodiment in which the core circuit CR of thedevice DIS is part of a band gap voltage generator BG.

The band gap voltage generator BG comprises the core circuit CR, andequalization circuit EG comprising an amplifier AMP whose non-invertinginput is coupled to the first node BE1 of the core circuit CR, and whoseinverting input is coupled to the second node BE2 of the core circuitCR, and a second output node BS2.

The equalization circuit EG comprise a coupling between the output ofthe amplifier AMP, coupled here to the second output node BS2, and theintermediate node NI via a switch INT controlled by the control signalCTRL. The switch INT is configured to be in an open state when thecontrol signal CTRL is in its first state, and in a closed state whenthe control signal CTRL is in its second state.

Likewise, the amplifier AMP can be activated by the control signal CTRLand is configured to be activated when the control signal CTRL is in itssecond state.

In operation, when the power supply voltage reaches the first threshold,the first comparator CMP1 delivers the control signal CTRL in its secondstate at a first output node BS1, the amplifier AMP is activated, andthe switch INT is closed. The second output node BS2 therefore deliversa band gap voltage Vbg.

In order to avoid oscillation phenomena when the value of the powersupply voltage crosses the voltage threshold, the first comparator CMP1is, here, a hysteresis comparator.

It is thus possible to use an existing band gap voltage generator corein the integrated circuit to produce the device DIS. This advantageouslyallows a surface saving on the device, compared to a circuit comprisinga band gap voltage generator which would be separate from the deviceDIS.

That being the case, the first comparator CMP1, the follower amplifierAS, and the amplifier AMP operate simultaneously, but, since theamplifier AMP has a higher gain, it imposes the band gap voltage on theintermediate node NI, here 1.2 Volts. The voltages at the first node BE1and at the second node BE2 are therefore kept equal by the amplifierAMP.

Thus, if the power supply voltage Vcc decreases, for example so as topass once again below the first threshold, the voltages at the first andsecond nodes BE1 and BE2 do not vary, and therefore the output of thefirst comparator CMP1 is not able to deliver the control signal CTRL inits first state.

It is not therefore possible to detect a drop in the power supplyvoltage Vcc in which the power supply voltage Vcc would once again passbelow the first threshold. In order to be able to detect such a voltagedrop, it is possible, as illustrated by FIG. 4, for the comparisoncircuit MC to comprise, in place of the first comparator CMP1, a secondcomparator CMP2 of which a first input E21 is coupled to the first nodeBE1, a second input E22 is coupled to the second node BE2, and a thirdcomparator CMP3, of which a first input E31 is coupled to the secondoutput node BS2, and of which a second input E32 is coupled to the powersupply node BV so as to receive the power supply voltage Vcc.

In order to avoid oscillation phenomena when the value of the powersupply voltage crosses the voltage threshold, the second and thirdcomparators CMP2 and CMP3 are, here, hysteresis comparators.

Furthermore, the amplifier AMP and the third comparator CMP3 can beactivated by the control signal CTRL.

In particular, when the control signal CTRL is in its first state, theamplifier AMP and the third comparator CMP3 are deactivated, and whenthe control signal CTRL is in its second state, the amplifier AMP andthe third comparator CMP3 are activated.

The second comparator CMP2 is configured to deliver a first signal SIG1having a first state, here a low state, if the power supply voltage islower than the first voltage threshold, that is to say if the voltage atthe first node BE1 is lower than the voltage at the second node BE2, andhaving a second state, here a high state, if the power supply voltage isgreater than or equal to the first voltage threshold, that is to say ifthe voltage at the first node BE1 is greater than or equal to thevoltage at the second node BE2.

The third comparator CMP3 is configured to, when it is activated,deliver the second signal SIG2 having a first state, here a low state,if the voltage on its second input E32, here the power supply voltageVcc, is greater than or equal to the voltage on its second input, herethe voltage on the second output node BS2, and to deliver the secondsignal SIG2 in a second state, here a high state, if the voltage on itssecond input E32 is lower than the voltage on its first input E31.

When the third comparator CMP3 is deactivated, its output delivers a lowstate. For example, the output of the third comparator CMP3 can becoupled to a circuit comprising pull-down transistors that can beactivated by the control signal CTRL.

The second comparator CMP2 and the third comparator CMP3 here each havetheir output coupled to the input of a logic gate PL.

The logic gate PL has its output coupled to the first output node BS1 ofthe device DIS, and is configured to deliver the control signal CTRL.

In operation, for example on start-up of the circuit CI, the powersupply voltage Vcc increases progressively to its maximum value.

In a first phase, during which the power supply voltage Vcc increaseswhile remaining below the first threshold, the voltage at the first nodeBE1 is lower than the voltage at the second node BE2, and the secondcomparator CMP2 delivers the first signal SIG1 in its first state.

The third comparator CMP3 is deactivated and delivers a low state. Theinverting gate PI therefore delivers a high state at the input of thelogic gate PL.

Thus, the states of the two inputs of the logic gate PL of AND type aredifferent, the logic gate PL therefore delivers the control signal CTRLin its first state.

In a second phase, beginning when the first voltage threshold isreached, that is to say when the voltages at the first and second nodesBE1 and BE2 are equal, the second comparator CMP2 delivers the firstsignal SIG1 having a high state.

Thus, the inputs of the logic gate PL are both in the high state, andthe logic gate PL, the output of which is coupled to the first outputnode BS1, delivers the control signal CTRL having a high state.

On reception of the control signal CTRL having the second value, thecircuit CI begins its operation, the switch INT switches to the closedposition, and the amplifier AMP and the third comparator CMP3 areactivated.

Thus, the first input E31 of the third comparator CMP3, coupled to thesecond output node BS2, receives the band gap voltage Vbg, which is hereequal to the first threshold, and which is therefore in the second phaselower than the power supply voltage Vcc received on the second inputE32.

The third comparator CMP3 therefore continues to deliver the secondsignal SIG2 having a low state.

Since the amplifier AMP has a higher gain, it imposes the band gapvoltage on the intermediate node NI, here 1.2 Volts. And, if the powersupply voltage Vcc decreases to pass once again below the firstthreshold, then the third comparator CMP3 delivers the second signalSIG2 having a high state.

The inverting gate PI therefore delivers a high state, and the inputs ofthe logic gate PL of AND type are therefore different.

The logic gate PL therefore delivers the control signal CTRL having thefirst state, here a low state, and the circuit CI then ceases itsoperation.

It is also possible to define a second threshold, lower than the firstvoltage threshold, used when the power supply voltage drops.

That advantageously makes it possible to avoid phenomena of oscillationof the device when the value of the power supply voltage reaches thevoltage threshold, and therefore make the device more stable.

As FIG. 5 illustrates, for that, the voltage power supply module MAshould comprise the first voltage divider bridge DIV1 in order that thefirst threshold is greater than the band gap voltage.

Here, the second input E32 of the third comparator CMP3 is coupled tothe power supply node BV via a second voltage divider bridge DIV2delivering a voltage equal to the power supply voltage Vcc divided by afirst factor.

The second voltage divider bridge DIV2 here comprises a first bridgeresistor R11 and a second bridge resistor R12, the values of which arechosen such that the second voltage threshold multiplied by said firstfactor is equal to the band gap voltage.

It would however be possible to choose a second threshold equal to theband gap voltage, and in this case the second input E32 of the thirdcomparator CMP3 would be directly coupled to the power supply node BV.

The invention is not limited to the embodiments which have just beendescribed but encompasses all the variants.

Thus, the invention is compatible with any core structure and anyvoltage power supply module structure.

The invention claimed is:
 1. An electronic circuit, comprising: a powersupply node configured to receive a power supply voltage; an outputnode; and a device configured to monitor the power supply voltagecomprising: a band gap voltage generator core coupled to the powersupply node and including a first node and a second node; a controlcircuit connected to the first and second nodes of the band gap voltagegenerator core and configured to deliver, on the output node, a controlsignal having: a first state when an increasing power supply voltage isbelow a first threshold; a second state when the increasing power supplyvoltage becomes greater than or equal to the first threshold, the firstthreshold being greater than a band gap voltage; and the first statewhen a decreasing power supply voltage falls below the first threshold;and a voltage power supply module configured to supply the band gapvoltage generator core at an intermediate node with a voltage derivedfrom the power supply voltage, wherein the voltage power supply moduleincludes a resistive module coupled between the power supply node andthe intermediate node.
 2. The circuit according to claim 1, wherein theintermediate node receives the power supply voltage.
 3. The circuitaccording to claim 1, wherein the band gap voltage generator corecomprises: first and second circuit branches respectively coupled to thefirst and second nodes and respectively comprising diode-mounted PNPbipolar transistors configured to exhibit different current densities; afirst resistor in one of the first and second circuit branches whichexhibits a higher current density; and two auxiliary resistorsrespectively connected to the first and second nodes and having a commonnode at an intermediate node which receives a voltage derived from thepower supply voltage.
 4. The circuit according to claim 1, wherein thecontrol circuit comprises: a first comparator having a first inputcoupled to the first node, a second input coupled to the second node,and an output coupled to the output node.
 5. The circuit according toclaim 1, further comprising an equalization circuit having a first inputcoupled to the first node, a second input coupled to the second node andan output coupled in feedback to the band gap voltage generator core,the equalization circuit being deactivated when the control signal is inits first state and being activated when the control signal is in itssecond state to deliver the band gap voltage at the output of theequalization circuit.
 6. The circuit according to claim 5, furthercomprising a switch coupled in series with the feedback, whereinactuation of said switch is controlled by the control signal.
 7. Thecircuit according to claim 5, where the output of the equalizationcircuit is coupled in feedback to the band gap voltage generator core atthe intermediate node.
 8. The circuit according to claim 5, wherein thecontrol circuit comprises: a first comparator having a first inputcoupled to the first node, a second input coupled to the second node,and an output is configured to generate a first signal when the powersupply voltage is greater than or equal to the first voltage threshold;a second comparator having a first input coupled to receive the band gapvoltage, a second input coupled to the power supply node, and an outputis configured to generate a second signal when the power supply voltageis greater than or equal to the first threshold; and a logic circuitconfigured to logically combine the first and second signals to generatethe control signal.
 9. The circuit according to claim 8, wherein thelogic circuit comprises: an inverter configured to invert the secondsignal; and an AND gate having a first input configured to receive thefirst signal and a second input configured to receive the invertedsecond signal.
 10. The circuit according to claim 8, wherein the controlcircuit is configured to deliver the control signal in the first statewhen a decreasing power supply voltage falls below a second thresholdthat is lower than the first threshold.
 11. The circuit according toclaim 10, wherein the second input of the second comparator is coupledto the power supply node via a voltage divider bridge, so that thesecond comparator is configured to generate the second signal when thepower supply voltage is greater than or equal to a second voltagethreshold, the second voltage threshold being different from the firstvoltage threshold.
 12. The circuit according to claim 1, wherein a firstvoltage and a second voltage are respectively generated at the first andsecond nodes from first and second currents passing through circuitry ofthe band gap voltage generator core having two different currentdensities.
 13. An electronic circuit, comprising: a power supply nodeconfigured to receive a power supply voltage; an output node; and adevice configured to monitor the power supply voltage comprising: a bandgap voltage generator core coupled to the power supply node andincluding a first node and a second node; a control circuit connected tothe first and second nodes of the band gap voltage generator core andconfigured to deliver, on the first output node, a control signalhaving: a first state when an increasing power supply voltage is below afirst threshold; and a second state when the increasing power supplyvoltage becomes greater than or equal to the first threshold, the firstthreshold being at least equal to a band gap voltage; and anequalization circuit having a first input coupled to the first node, asecond input coupled to the second node and an output coupled infeedback to the band gap voltage generator core, the equalizationcircuit being deactivated when the control signal is in its first stateand being activated when the control signal is in its second state todeliver the band gap voltage at the output of the equalization circuit.14. The circuit according to claim 13, further comprising a voltagepower supply module configured to supply the band gap voltage generatorcore at an intermediate node with a voltage derived from the powersupply voltage.
 15. The circuit according to claim 14, wherein theintermediate node receives the power supply voltage, and the firstthreshold is equal to the band gap voltage.
 16. The circuit according toclaim 14, wherein the voltage power supply module includes a resistivemodule coupled between the power supply node and the intermediate node,and the first voltage threshold is greater than the band gap voltage.17. The circuit according to claim 13, wherein the band gap voltagegenerator core comprises: first and second circuit branches respectivelycoupled to the first and second nodes and respectively comprisingdiode-mounted PNP bipolar transistors configured to exhibit differentcurrent densities; a first resistor in one of the first and secondcircuit branches which exhibits a higher current density; and twoauxiliary resistors respectively connected to the first and second nodesand having a common node at an intermediate node which receives avoltage derived from the power supply voltage.
 18. The circuit accordingto claim 13, wherein the control circuit comprises: a first comparatorhaving a first input coupled to the first node, a second input coupledto the second node, and an output coupled to the first output node. 19.The circuit according to claim 13, further comprising a switch coupledin series with the feedback, wherein actuation of said switch iscontrolled by the control signal.
 20. The circuit according to claim 13,where the output of the equalization circuit is coupled in feedback tothe band gap voltage generator core at an intermediate node whichreceives a voltage derived from the power supply voltage.
 21. Thecircuit according to claim 13, wherein the control circuit comprises: afirst comparator having a first input coupled to the first node, asecond input coupled to the second node, and a first output isconfigured to generate a first signal when the power supply voltage isgreater than or equal to the first voltage threshold; a secondcomparator having a first input coupled to receive the band gap voltage,a second input coupled to the power supply node, and a second output isconfigured to generate a second signal when the power supply voltage isgreater than or equal to the first threshold; and a logic circuitconfigured to logically combine the first and second signals to generatethe control signal.
 22. The circuit according to claim 21, wherein thelogic circuit comprises: an inverter configured to invert the secondsignal; and an AND gate having a first input configured to receive thefirst signal and a second input configured to receive the invertedsecond signal.
 23. The circuit according to claim 21, wherein thecontrol circuit is configured to deliver the control signal in the firststate when a decreasing power supply voltage falls below a secondthreshold that is lower than the first threshold.
 24. The circuitaccording to claim 23, wherein the second input of the second comparatoris coupled to the power supply node via a voltage divider bridge, sothat the second comparator is configured to generate the second signalwhen the power supply voltage is greater than or equal to a secondvoltage threshold, the second voltage threshold being different from thefirst voltage threshold.
 25. The circuit according to claim 13, whereina first voltage and a second voltage are respectively generated at thefirst and second nodes from first and second currents passing throughcircuitry of the band gap voltage generator core having two differentcurrent densities.
 26. An electronic circuit, comprising: a power supplynode configured to receive a power supply voltage; an output node; and adevice configured to monitor the power supply voltage comprising: a bandgap voltage generator core coupled to the power supply node andincluding a first node and a second node; a control circuit connected tothe first and second nodes of the band gap voltage generator core andconfigured to deliver, on the output node, a control signal having: afirst state when an increasing power supply voltage is below a firstthreshold; and a second state when the increasing power supply voltagebecomes greater than or equal to the first threshold, the firstthreshold being at least equal to a band gap voltage; wherein thecontrol circuit comprises: a first comparator having a first inputcoupled to the first node, a second input coupled to the second node,and a first output configured to generate a first signal when the powersupply voltage is greater than or equal to the first voltage threshold;a second comparator having a first input coupled to receive the band gapvoltage, a second input coupled to the power supply node, and a secondoutput configured to generate a second signal when the power supplyvoltage is greater than or equal to the first threshold; and a logiccircuit configured to logically combine the first and second signals togenerate the control signal at the output node.
 27. The circuitaccording to claim 26, wherein the logic circuit comprises: an inverterconfigured to invert the second signal; and an AND gate having a firstinput configured to receive the first signal and a second inputconfigured to receive the inverted second signal.
 28. The circuitaccording to claim 26, wherein the control circuit is configured todeliver the control signal in a first logic state when a decreasingpower supply voltage falls below a second threshold that is lower thanthe first threshold.
 29. The circuit according to claim 28, wherein thesecond input of the second comparator is coupled to the power supplynode via a voltage divider bridge, so that the second comparator isconfigured to generate the second signal when the power supply voltageis greater than or equal to a second voltage threshold, the secondvoltage threshold being different from the first voltage threshold. 30.The circuit according to claim 26, further comprising a voltage powersupply module configured to supply the band gap voltage generator coreat an intermediate node with a voltage derived from the power supplyvoltage.
 31. The circuit according to claim 30, wherein the intermediatenode receives the power supply voltage, and the first threshold is equalto the band gap voltage.
 32. The circuit according to claim 30, whereinthe voltage power supply module includes a resistive module coupledbetween the power supply node and the intermediate node, and the firstvoltage threshold is greater than the band gap voltage.
 33. The circuitaccording to claim 26, wherein the band gap voltage generator corecomprises: first and second circuit branches respectively coupled to thefirst and second nodes and respectively comprising diode-mounted PNPbipolar transistors configured to exhibit different current densities; afirst resistor in one of the first and second circuit branches whichexhibits a higher current density; and two auxiliary resistorsrespectively connected to the first and second nodes and having a commonnode at an intermediate node which receives a voltage derived from thepower supply voltage.
 34. The circuit according to claim 26, wherein afirst voltage and a second voltage are respectively generated at thefirst and second nodes from first and second currents passing throughcircuitry of the band gap voltage generator core having two differentcurrent densities.